In-line photoluminescence imaging of semiconductor devices

ABSTRACT

Methods and systems are presented for acquiring photoluminescence images ( 2 ) of silicon solar cells and wafers ( 4 ) as they progress along a manufacturing line ( 36 ). In preferred embodiments the images are acquired while maintaining motion of the samples. In certain embodiments photoluminescence is generated with short pulse, high intensity excitation, ( 8 ) for instance by a flash lamp ( 50 ) while in other embodiments images are acquired in line scanning fashion. The photoluminescence images can be analysed to obtain information on average or spatially resolved values of one or more sample properties such as minority carrier diffusion length, minority carrier lifetime, dislocation defects, impurities and shunts, or information on the incidence or growth of cracks in a sample.

FIELD OF THE INVENTION

The present invention relates to methods and systems for performingphotoluminescence analyses of semiconductor devices, and of siliconsolar cells in particular, during or after their production process.

RELATED APPLICATIONS

The present application claims priority from Australian provisionalpatent application Nos 2010900018, 2010903050 and 2010903975, thecontents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Any discussion of the prior art throughout this specification should inno way be considered as an admission that such prior art is widely knownor forms part of the common general knowledge in the field.

Photoluminescence (PL) imaging, performed for example using apparatusand methods disclosed in PCT Patent Application Publication No WO2007/041758 A1 entitled ‘Method and System for Inspecting IndirectBandgap Semiconductor Structure’ and incorporated herein by reference,has been shown to be of value for the rapid characterisation of siliconmaterials and devices, and silicon wafer-based solar cells inparticular. As shown schematically in FIG. 1, luminescence 2 generatedfrom a semiconductor sample 4 with broad area photo-excitation from asource 6 of above-bandgap light 8 can be imaged with a camera or CCDarray 10 via collection optics 11, with the system preferably includinghomogenisation optics 12 to improve the uniformity of the broad areaexcitation and a long-pass filter 14 in front of the camera to blockexcitation light. The system may also include one or more filters 15 toselect the wavelength range of the photo-excitation. With relativelythin samples it is also possible to have the excitation source 6 andcamera 10 on opposite sides of the sample 4 as shown in FIG. 2, in whichcase the sample itself can serve as a long-pass filter. However along-pass filter 14 may still be required if a significant amount ofstray excitation light, reflected for example off other components, isreaching the camera. Either way, the acquired PL image can be analysedwith a computer 16, using techniques disclosed for example in publishedPCT patent application Nos WO 2008/014537 A1, WO 2009/026661 A1 and WO2009/121133 A1, to obtain information on average or spatially resolvedvalues of a number of sample properties including minority carrierdiffusion length, minority carrier lifetime, dislocation defects,impurities and shunts, amongst others, or on the incidence or growth ofcracks. In principle the entire process can be performed in a matter ofseconds or fractions of a second depending on factors such as thequality of silicon material and the readout speed of the camera, whichis a timescale generally compatible with current solar block, cell andwafer production lines where, for example, the throughput for wafer andcell lines is of order one cell per second or two, and for blockproduction where 30 seconds is typically available for the measurementof a full block face.

However the current PL imaging system as described above suffers from anumber of disadvantages.

One disadvantage is that currently available PL imaging systems requiresamples to be removed from production lines and taken to the PL imagingtool, for example using robotic or manual pick and place handling.Manual pick and place handling is a labour intensive and slow processthat often adds cost as well as being slow, and although robotic pickand place handling systems using platens or suction cups or similar aresomewhat faster, they also add cost. Either way, the limited speed meansonly a small sample of the product in process can be tested. It would bebeneficial to be able to measure all or most of the work product.

A further disadvantage is that in current PL imaging systems the samplehas to be stationary during the measurement to prevent blurring of theimage. A blurred image can prevent or compromise the capture ofspatially resolved characterisation data, complicating the design and/orincorporation of a PL imaging system into production lines that areincreasingly operating in continuous mode without stopping. To explain,with broad area 1 Sun excitation the photoluminescence emitted from manysilicon samples, and raw or unpassivated silicon samples in particular,can be of such low intensity that even the most sensitive commerciallyavailable silicon-based CCD cameras require an exposure time of order atleast 0.5 second to acquire a sufficient PL signal.

Yet another disadvantage with current PL imaging systems is the commonreliance on laser excitation sources, typically in the near IR region ofthe spectrum. To explain, obtaining a measurable PL signal from lowphotoluminescence quantum efficiency samples such as raw or unpassivatedsilicon wafers and blocks (with quantum efficiency of order 10⁻⁶) oftenrequires illumination intensities of 0.1 Watts/cm² (˜1 Sun) or greater.A total optical power of tens of Watts is therefore required toilluminate silicon solar cell wafers that may typically be 15.6×15.6 cm²in area, and laser excitation sources are usually considered to beessential to provide the required spectral purity and beam shaping.Furthermore for silicon samples the excitation light is typically in thenear IR region (750 to 1000 nm), which is potentially very harmfulbecause the eye focuses near-infrared light onto the retina but itsprotective ‘blink reflex’ response is triggered only by visible light.The potential hazard of laser light sources arises from the fact thatthey may be much brighter than other light sources, where the brightness(in units of power per unit area per unit solid angle) may be definedfor example as the optical power passing through an aperture (e.g. alaser output aperture) divided by the aperture area divided by the solidangle subtended by the optical beam in the far field. When an extremelybright light source is viewed with the eye, either directly or viaintermediate optics such as a collimating lens, the image formed on theretina can be extremely intense, resulting in virtually instantaneousand permanent damage. However although there is less likelihood of thisoccurring with incoherent near IR light, e.g. from high power LEDs, itneeds to be understood that because brightness is a key parameter, lightsafety issues cannot simply be ignored just because a system usesnon-laser (incoherent) light sources.

Current PL imaging systems are therefore further complicated by lightsafety issues, since the PL measurement chamber generally must beoptically isolated to avoid the risk of operators being exposed to highbrightness IR light that could cause eye damage. This usually requiresshutters, doors or equivalent mechanisms, adding complexity and cost tothe sample transfer mechanisms into and out of the PL measurementchamber. Because of these complications, the basic PL imaging apparatusshown in FIG. 1 or 2 requires several modifications if it is to be usedsafely and cost effectively to characterise silicon solar cells on aproduction line.

SUMMARY OF THE INVENTION

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative. It is an object of a preferred form of the presentinvention to provide methods and systems for acquiring photoluminescenceimages of semiconductor devices during their production process withoutremoving them from the production line. It is another object of apreferred form of the present invention to provide methods and systemsfor acquiring photoluminescence images of semiconductor devices duringtheir production process without interrupting the motion of the devicesthrough the production line. It is another object of a preferred form ofthe present invention to provide methods and systems for acquiringphotoluminescence images of semiconductor devices using imaging systemsthat are eye-safe without requiring light safety shutters. It is anotherobject of a preferred form of the present invention to provide methodsand systems for acquiring photoluminescence images of silicon wafers orcells with a total measurement time between 0.1 and 1 second per waferor cell. It is another object of a preferred form of the presentinvention to provide a photoluminescence imaging system that does nothave or require an integrated sample handling stage.

According to a first aspect the invention provides a method foranalysing a sample of a semiconductor material, said method comprisingthe steps of:

passing said sample to a measurement zone;

applying an illumination to said semiconductor material to produce aphotoluminescence response; and

conducting at least a photoluminescence analysis of said sample in saidmeasurement zone while maintaining motion of said sample.

According to a second aspect of the invention provides a method ofconducting a photoluminescence analysis of a sample of a semiconductormaterial moving through a measurement zone, said method comprising thesteps of: applying an illumination to the semiconductor material for asufficient time and intensity to produce a photoluminescence response;and capturing an image of the photoluminescence emanating from saidsemiconductor material, wherein said image capture is obtained within adistance of up to 1 or 2 pixels of the imaging camera.

Preferably, said photoluminescence analysis comprises the steps of:

illuminating an area of said sample with a predetermined illumination togenerate photoluminescence from said sample in response to saidillumination; and

acquiring an image of said photoluminescence with an area image capturedevice in an acquisition time t,

wherein said sample is moving at a speed v relative to said area imagecapture device, and wherein the product of the acquisition time t (s)and the speed v (m·s⁻¹) is less than a distance on said samplecorresponding to one row of pixels in said image capture device.

Preferably said illumination comprises incoherent light. Saidillumination may comprise a pulse of light.

Preferably, said photoluminescence analysis comprises the steps of:

providing a source of predetermined illumination suitable for generatingphotoluminescence from said sample, said source being positioned so asto illuminate a first portion of said sample;

providing an image capture device for detecting the photoluminescenceemitted from a second portion of said sample, wherein said first portionand said second portion are at least partially overlapping;

moving said sample relative to said source and to said image capturedevice such that said second portion is scanned across a substantialarea of said sample; and

interrogating said image capture device repeatedly to acquire an imageof the photoluminescence emitted from said area.

Preferably said illumination comprises incoherent light. Preferably theimage capture device is a line camera.

Preferably said first portion is from one to five times wider than saidsecond portion in the direction of movement of said sample.

Alternatively, the image capture device is a time-delayed integrationcamera.

In one embodiment said first portion is substantially coterminous withsaid second portion. In another embodiment, said first portion is whollyor partially within said second portion. In yet another embodiment, saidsecond portion is wholly or partially within said first portion.

Preferably, said first and second portions extend across a substantialfraction of a dimension of said sample, said dimension beingsubstantially perpendicular to the direction of movement of said sample.

The semiconductor material may be raw or unpassivated silicon. In thatcase, it is preferred if said photoluminescence is generated with anillumination intensity between about 1 and 40 W·cm⁻².

Alternatively, said semiconductor material may be passivated silicon. Inthat case, it is preferred if said photoluminescence is generated withan illumination intensity between about 0.1 and 10 W·cm⁻².

Preferably the illumination source and/or an optical element associatedtherewith moves within the measurement zone. In that case, it ispreferred that motion of the illumination source and/or an opticalelement associated therewith is controlled to maintain a predeterminedalignment with a sample. The predetermined alignment may be to avoidblurring of illumination of the sample.

Preferably, the image capture device and/or an optical elementassociated therewith moves within the measurement zone. In that case, itis preferred that motion of the image capture device and/or an opticalelement associated therewith is controlled to maintain a predeterminedalignment with a sample. The predetermined alignment may be to avoidblurring of image capture of the sample.

Preferably, illumination is introduced into the imaging optical systemusing a dichroic mirror. In that case, it is preferred ifphotoluminescence data captured passes through said dichroic mirror.

Preferably said photoluminescence analysis provides information onaverage or spatially resolved values of one or more properties of saidsample, said properties being selected from the group consisting ofminority carrier diffusion length, minority carrier lifetime,dislocation defects, impurities and shunts.

Preferably said sample is a silicon wafer.

Preferably said analysis is performed in less than 1 second.

In one embodiment, the configuration of the measurement zone remainsconstant before, during and after data acquisition. The measurement zonemay be a shuttered or enclosed chamber. Alternatively, said measurementzone is unenclosed.

According to a third aspect the invention provides a method foranalysing a sample of a semiconductor material, said method comprisingthe steps of:

passing said sample to a measurement zone; and

conducting at least a photoluminescence analysis of said sample in saidmeasurement zone, wherein said measurement zone is eye-safe withoutbeing enclosed.

According to a fourth aspect the invention provides a method foranalysing a sample of a semiconductor material, said method comprisingthe steps of:

passing said sample to a measurement zone without using a pick and placesample handling system; and

conducting at least a photoluminescence analysis of said sample in saidmeasurement zone.

According to a fifth aspect the invention provides a system forconducting an analysis of a sample of a semiconductor material, saidapparatus comprising:

a transport mechanism for transporting said sample to a measurementzone;

analysis equipment for conducting at least a photoluminescence analysisof said sample within said measurement zone; and

motion apparatus to maintain motion of said sample within saidmeasurement zone during said analysis.

Preferably said analysis equipment comprises:

an optical source for illuminating an area of said sample with apredetermined illumination to generate photoluminescence from saidsample in response to said illumination; and

an area image capture device for capturing an image of saidphotoluminescence in an image acquisition time t, wherein said motionapparatus moves said sample at a speed v relative to said area imagecapture device such that the product of the image acquisition time t (s)and the speed v (m·s⁻¹) is less than a distance on said samplecorresponding to one row of pixels in said area image capture device.

Preferably said analysis equipment comprises:

a source of predetermined illumination suitable for generatingphotoluminescence from said sample, said source being positioned so asto illuminate a first portion of said sample;

an image capture device for detecting the photoluminescence generatedfrom a second portion of said sample, wherein said first portion andsaid second portion are at least partially overlapping; and

means for interrogating said image capture device repeatedly while saidmotion means moves said sample such that said second portion is scannedacross a substantial area of said sample, to acquire an image of thephotoluminescence emitted from said area.

Preferably the illumination source and/or an optical element associatedtherewith moves within the measurement zone. In that case, it ispreferred that motion of the illumination source and/or an opticalelement associated therewith is controlled to maintain a predeterminedalignment with a sample. The predetermined alignment may be to avoidblurring of illumination of the sample.

Preferably, the image capture device and/or an optical elementassociated therewith moves within the measurement zone. In that case, itis preferred that motion of the image capture device and/or an opticalelement associated therewith is controlled to maintain a predeterminedalignment with a sample. The predetermined alignment may be to avoidblurring of image capture of the sample.

Preferably, illumination is introduced into the imaging optical systemusing a dichroic mirror. In that case, it is preferred if thephotoluminescence passes through said dichroic mirror prior to imagecapture.

Preferably the configuration of the measurement zone remains constantbefore, during and after data acquisition.

In one embodiment, said measurement zone is a shuttered or enclosedchamber. In an alternative embodiment said measurement zone isunenclosed.

According to a sixth aspect the invention provides a system forconducting an analysis of a sample of a semiconductor material, saidsystem comprising:

a transport mechanism for transporting said sample to a measurementzone;

a light source for illuminating an area of said sample to generatephotoluminescence from said sample; and

analysis equipment for conducting at least a photoluminescence analysisof said sample within said measurement zone, wherein said measurementzone is eye-safe without being enclosed.

According to a seventh aspect the invention provides a system forconducting an analysis of a sample of a semiconductor material, saidsystem comprising:

a transport mechanism for transporting said sample to a measurement zonewithout using a pick and place sample handling system; and

analysis equipment for conducting at least a photoluminescence analysisof said sample in said measurement zone.

According to a eighth aspect the invention provides a method ofanalysing a sample of semiconductor material from within a series ofsamples of semiconductor material, said method comprising the steps of:

passing said sample to a measurement zone; and

acquiring photoluminescence data from said sample in said measurementzone while enabling motion of other samples in said series of samples.

In one embodiment said sample is in motion during acquisition ofphotoluminescence data.

In an alternative embodiment said sample is stationary duringacquisition of photoluminescence data.

The other samples in said series of samples may be in motion duringacquisition of photoluminescence data, or alternatively, the othersamples in said series of samples are not in motion during acquisitionof photoluminescence data.

According to a ninth aspect the invention provides a method of analysinga sample of semiconductor material, said method comprising the steps of:

conveying said sample to a point adjacent to a measurement zone by adelivery transport means;conveying said sample into, through and out of said measurement zone bya measurement zone transport means;acquiring photoluminescence data from said sample in said measurementzone; andconveying said sample from a point adjacent the measurement zone by aremoval transport means, wherein the measurement zone transport means iscontrollable independently of said delivery transport means or saidremoval transport means.

In one embodiment, the measurement zone transport means and/or sample ismotionless during the acquisition of photoluminescence data.

In an alternative embodiment, the measurement zone transport meansand/or sample is in motion during the acquisition of photoluminescencedata.

Preferably said photoluminescence is generated by illumination withincoherent light.

Preferably said photoluminescence is generated by illuminationcomprising a pulse of light.

In one embodiment, analysing a sample of semiconductor material takesplace in a shuttered or enclosed analysis chamber.

In an alternative embodiment, analysing a sample of semiconductormaterial takes place in an unshuttered or at least partially openanalysis chamber.

Preferably the sample is illuminated by illumination introduced using adichroic mirror.

Preferably the acquired photoluminescence data is analysed to obtaininformation on average or spatially resolved values of a sample propertyselected from the group consisting of minority carrier diffusion length,minority carrier lifetime, dislocation defects, impurities and shunts.

Preferably said sample is a silicon wafer.

Preferably said analysis is performed in less than 1 second.

According to a tenth aspect the invention provides a method foranalysing a sample of a semiconductor material in a measurement zone,said method comprising a transport mechanism for moving and supportingsaid sample, wherein said transport mechanism contacts no more than 10%of said sample, thereby leaving 90% of said sample exposed at all timesfor analysis.

According to a eleventh aspect the invention provides a method foranalysing a sample of a semiconductor material in a measurement zone,said method comprising a transport mechanism for movement and support ofsaid sample during analysis wherein during said analysis at least aportion of said sample is left unsupported across its entire width toprovide an unobstructed region for said analysis whereby, as a result ofcontinued movement, the entire sample is progressively unobstructed.

According to a twelfth aspect the invention provides a system foranalysing a sample of semiconductor material, said system comprising:

a delivery transport means to convey said sample to a point adjacent toa measurement zone;a measurement zone transport means for conveying said sample into,through and out of said measurement zone;an illumination means for generating photoluminescence from said samplein said measurement zone;a detector for detecting said photoluminescence; anda removal transport means for conveying said sample from a pointadjacent the measurement zone, wherein the measurement zone transportmeans is controllable independently of said delivery transport means orsaid removal transport means.

Preferably said illumination means emits incoherent light.

Preferably said illumination means emits a pulse of light.

Preferably the delivery transport means, measurement zone transportmeans and removal transport means are independently selected from thegroup consisting of a belt, a series of rollers, a series of platens, aplurality of aligned belts and a vacuum chuck.

Preferably the measurement zone transport means is a plurality ofaligned belts configured to support opposed sides of a semiconductorsample, the area between said belts defining an unobstructed centralportion of the semiconductor sample.

Preferably light from the illumination means is introduced to the sampleusing a dichroic mirror.

In one embodiment, analysing a sample of semiconductor material takesplace in a shuttered or enclosed analysis chamber.

In an alternative embodiment, analysing a sample of semiconductormaterial takes place in an unshuttered or at least partially openanalysis chamber.

Preferably the delivery transport means, the measurement zone transportmeans and the removal transport means do not use a pick and place samplehandling system.

Preferably an acquired photoluminescence image is analysed to obtaininformation on average or spatially resolved values of a sample propertyselected from the group consisting of minority carrier diffusion length,minority carrier lifetime, dislocation defects, impurities and shunts.

Preferably said sample is a silicon wafer.

Preferably said analysis is performed in less than 1 second.

According to a thirteenth aspect the invention provides a system foranalysing a sample of a semiconductor material in a measurement zone,said system comprising a transport mechanism for moving and supportingsaid sample, wherein said transport mechanism contacts no more than 10%of said sample, thereby leaving 90% of said sample exposed at all timesfor analysis.

According to a fourteenth aspect the invention provides a system foranalysing a sample of a semiconductor material in a measurement zone,said system comprising a transport mechanism for movement and support ofsaid sample during analysis wherein during said analysis at least aportion of said sample is left unsupported across its entire width toprovide an unobstructed region for said analysis whereby, as a result ofcontinued movement, the entire sample is progressively unobstructed.

According to a fifteenth aspect, the present invention provides aproduction line for the production of a photovoltaic device, saidproduction line comprising a plurality of process steps to convert asemiconductor material to said photovoltaic device, said production lineincluding at least one analysis device comprising a illumination sourcefor application to a semiconductor material, and a non-stop imagecapture device for obtaining an image of photoluminescence emanatingfrom said illuminated semiconductor material without stopping thesemiconductor material.

According to a sixteenth aspect, the present invention provides aproduction line for the production of photovoltaic device, saidproduction line comprising a plurality of process steps to convert asemiconductor material to said photovoltaic device, said production lineincluding at least one eye-safe analysis device having a high intensityillumination system for applying illumination with an intensity ofgreater than 10 Suns to the semiconductor material, and an image capturedevice for obtaining an image of photoluminescence emanating from saidilluminated semiconductor material, said analysis device being adaptedto illuminate said semiconductor material and capture an image ofphotoluminescence emanating from said illuminated semiconductor materialwithout having to stop said semiconductor material as it moves throughsaid production line.

BRIEF DESCRIPTION OF THE DRAWINGS

Benefits and advantages of the present invention will become apparent tothose skilled in the art to which this invention relates from thesubsequent description of exemplary embodiments and the appended claims,taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a prior art system for PL imaging of a semiconductorsample;

FIG. 2 illustrates another prior art system for PL imaging of asemiconductor sample;

FIG. 3 illustrates a system suitable for in-line PL imaging ofsemiconductor samples;

FIG. 4 shows in plan view the positioning of a semiconductor sample on atransport belt;

FIG. 5 illustrates in side view a PL imaging system according to anembodiment of the invention;

FIG. 6 shows in plan view an arrangement of a flash lamp and cameraaccording to a preferred embodiment of the invention;

FIG. 7 shows in side view a system for PL imaging of a semiconductorsample;

FIGS. 8A and 8B show in plan view and side view respectively a linecamera system for acquiring PL images from a continuously moving sample;

FIG. 9 shows in side view another line camera system for acquiring PLimages from a continuously moving sample;

FIGS. 10A and 10B show in side view and plan view respectively anilluminator for a line camera PL system;

FIGS. 11A and 11B show in side view and plan view respectively a systemof collection optics for a line camera PL system;

FIG. 12 shows another system of collection optics for a line camera PLsystem; and

FIG. 13 shows in side view a TDI camera system for acquiring PL imagesfrom a continuously moving sample.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawings.

FIG. 3 shows a PL imaging system according to one embodiment of thepresent invention, suitable for acquiring photoluminescence images ofsemiconductor devices during their production process without removingthem from the production line. This system, hereinafter referred to as a‘three-belt system’, includes two outer transport belts 18 to interfacewith a continuous on-belt production line or pick and place handlingrobots or other handling mechanism common to a production line and aninner transport belt 20 to bring the sample 4 to a stop for measurement,and optionally light-tight shutters 22 that open to allow the samples inand out of the measurement chamber 24 to satisfy light safetyrequirements if required, i.e. if the system would not otherwise beeye-safe. In alternative embodiments the sample is moved with some othertransport mechanism such as rollers, platens or vacuum chucks ratherthan transport belts.

Although the apparatus shown in FIG. 3 has the excitation source 6 andcamera 10 on the same side of the sample as in FIG. 1, the FIG. 2arrangement is also possible because, as shown in plan view in FIG. 4,the inner transport belt 20 at least can be in split form, allowing asubstantial portion 26 of the sample 4 to be illuminated and/or imaged.By way of concrete example, for a 15.6 cm×15.6 cm wafer supported on apair of 5 mm wide transport belts, up to 93% of the wafer surface areacan be available for illumination or imaging.

This three-belt system is a major advancement over the current PLimaging sampling systems, since it enables measurement of all or nearlyall samples in a production process, subject to both line and toolmeasurement speed. In this context, it alleviates one of the majordisadvantages of current PL imaging systems.

However it would be advantageous, especially with fragile wafers, or onlarge or fast production lines, to avoid any stop/starting of individualwafers, especially since the rate of stopping and starting becomesthroughput limiting as overly rapid acceleration or de-acceleration ofsamples on a belt may lead to breakages or sample slip. It would beparticularly advantageous to have a system that can not only measure allor most of the samples in a production process, but that does notrequire the sample to stop for measurement, and requires few or no lightsafety measures. In addition the absence of an expensive sample handlingapparatus, for example using robotic pick and place sample handlingutilising platens, suction cups or the like, integrated into the PLmeasuring system would be of economic benefit.

Specifically, it would be especially advantageous to have a PL imagingsystem that just included a camera, a light source and optics as themajor hardware components. Such a system could be placed anywhere in aproduction line without requiring special modifications, for exampleabove a transport belt bearing samples along the line. The invention isnow described with reference to various systems and methods foracquiring PL images of semiconductor devices, such as fully or partiallymanufactured silicon solar cells, during their production processwithout interrupting their motion through the production line. Theinvention is described with reference to configurations where theimaging device and illuminator are stationary with the samples movingpast them, for example, on a system of transport belts or robotgrippers, however it should be noted that other configurations are alsowithin the scope of the present invention. For example blurring can beavoided during long exposures by moving or pivoting the imaging deviceand/or its associated optics to follow the sample movement. In someembodiments the excitation source and/or its associated optics can alsobe moved or pivoted to follow the sample movement.

Before describing further embodiments of PL imaging systems, and inparticular those preferred embodiments with reduced or no light safetyrequirements, it will be useful to include some discussion of currentlaser safety standards and some strategies for producing PL imagingsystems with reduced light safety requirements. As mentioned previouslythe brightness of a source, which can be defined as the optical powerpassing through an aperture divided by the aperture area divided by thesolid angle subtended by the optical beam in the far field, is a keyparameter, and light safety issues cannot simply be ignored just becausea system uses non-laser (incoherent) light sources.

In Australia and New Zealand, the standards for laser classification andsafety requirements are provided by AS/NZS 2211.1:2004 and itsassociated guidelines (AS/NZS 2211.10:2004), based on the internationalstandard IEC 60825-1:2001. An important concept in laser safety is the‘Maximum Permissible Exposure’ (MPE) level, which is defined in thestandard as ‘that level of laser radiation to which, under normalcircumstances, persons may be exposed without suffering adverseeffects’. The definition further states that ‘MPE levels represent themaximum level to which the eye or skin can be exposed withoutconsequential injury immediately, or after a long time, and are relatedto the wavelength of the radiation, the pulse duration or exposure time,the tissue at risk and, for visible and near infra-red radiation in therange of 400 nm to 1 400 nm, the size of the retinal image’.

Since the wavelengths of light suitable for generating PL from siliconare within this 400 to 1400 nm range, it follows that retinal image sizeis a key factor for light safety in PL imaging systems. Within certainlimits, the MPE level increases with increased image size on the retina,although there is no decrease in the MPE below a certain minimum imagesize and no increase above a certain maximum image size. Forquantitative purposes the standard uses an angular measure of theretinal image size, the angle subtended by the source at the eye, α.This is generally referred to as the ‘angular subtense’ and is givenapproximately by the source size divided by the distance between thesource and the eye. The angular subtense representing the image sizebelow which there is no further decrease in the MPE is referred to as‘α_(min)’ (1.5 mrad), and exposure conditions below this are referred toas ‘point source viewing’. ‘Extended source viewing’ conditions apply atangular subtenses above α_(min), and as the angular subtense increasesfrom α_(min) the MPE level increases until it reaches a maximum atα=α_(max) (100 mrad), beyond which the MPE is constant. It is importantto note that if the source radiation is modified by illumination optics,as shown in FIGS. 1-3 for example, the ‘apparent source’ for MPEpurposes is the image, real or virtual, that produces the smallestretinal image. For the purposes of this specification, the term‘illuminator’ will be used to refer to the portion of a PL imagingsystem that provides optical excitation to a sample. An illuminator willinclude one or more optical sources, possibly in combination with anumber of other components including filters and focusing optics.

In the standards, laser products are classified in a system ranging fromClass 1, ‘safe under reasonably foreseeable conditions of operation’, toClass 4, ‘generally powerful enough to burn skin and cause fires’, usinglimits known as ‘accessible emission limits’ (AELs). AELs are derivedfrom MPEs using limiting apertures and may be expressed as a powerlimit, an energy limit, an irradiance limit, a radiant exposure limit,or a combination thereof. The limiting aperture is usually taken to be 7mm, representing a dilated pupil as a ‘worst case scenario’. Althoughmeeting Class 1 AELs is necessary but not sufficient for making a laserproduct Class 1, there being other constraints, for the purposes of thisspecification a PL imaging system as a whole will be considered to be‘eye-safe’ if it meets Class 1 AELs. Similarly, the illuminator portionof an imaging system will be considered to be ‘eye-safe’ if it meetsClass 1 AELs.

Relatively high brightness sources, typically required for acquiring PLimages of silicon PV samples on a timescale suitable for in-lineapplications, are potentially hazardous because they can result in arelatively high intensity at the eye, even at a distance, or arelatively small retinal image (and correspondingly low MPE level).However to determine the actual hazard, it is necessary to considerbrightness in combination with the viewing conditions, in particular theangular subtense. The importance of viewing conditions is demonstratedby the following specific example. According to the calculationmethodology prescribed in IEC 60825-1:2001, an 808 nm cw laser productcan only be classified as Class 1 (i.e. does not exceed the Class 1 AEL)if its emission under point source viewing conditions (i.e. angularsubtense α<α_(min)) does not exceed 0.64 mW through a 7 mm diameterlimiting aperture. In contrast, for extended source viewing conditionswhere α>α_(max) (100 mrad), the Class 1 AEL is 42 mW (i.e. 65× higher)through a 7 mm diameter limiting aperture.

The brightest light sources in common use are laser sources, which havehigh temporal coherence (or equivalently, coherence length) compared tonon-laser (i.e. thermal) sources. Since coherence is an inherent aspectof the lasing process, higher coherence than thermal sources may beconsidered a necessary condition for achieving the highest brightnesspractical light sources. However coherence does not imply brightness, asit is not a sufficient condition. In general, coherence length varieswidely (over orders of magnitude) between different laser types, butthis does not necessarily correlate with brightness. For example thecoherence length of a laser source can be increased by using a highquality factor (Q) resonator at the expense of output power, meaningthat while the beam collimation (the ‘per unit solid angle’ part of thebrightness definition) may be increased, the reduced output powerreduces the ‘power per unit area’ part of the brightness definition,counteracting the potential increase in brightness.

Optics can be added to a light source to reduce the brightness withoutaltering the coherence, a trivial example being an absorbing filterwhich may be used to reduce the brightness arbitrarily without alteringthe coherence. Of significant practical relevance for PL imaging systemsof the present invention are illuminator designs which have reducedbrightness without significantly reducing the intensity on the sample,typically all or part of a wafer or PV cell. In certain embodiments thisis achieved in a second illuminator (‘system 2’) compared to anunimproved, prior art illuminator (‘system 1’) by one or both of:

(i) Increasing the solid angle filled by the light output from system 2relative to that of system 1. This may be expressed as decreasing the ‘fnumber’ or increasing the Numerical Aperture of the illuminator, andessentially the excitation light is made to diverge more rapidly so thatits intensity at a distance is reduced.(ii) Increasing the size of the source (real or apparent, as discussedabove in the context of illumination optics) in system 2, for example bydividing a single beam in system 1 into one or more beams, or an arrayof beamlets, in system 2, or by mechanically agitating a component ofthe illumination system (e.g. a mirror). If system 1 already uses anumber of beamlets, their number may be significantly increased insystem 2.

Approach (i) decreases the intensity of light at the eye, while approach(ii) increases the angular subtense a which, subject to the limitsdescribed above, may increase the MPE level as follows:

(a) If α for system 1 was greater than α_(min) and less than α_(max),then the MPE level for system 2 is greater than for system 1.(b) If α for system 1 was less than α_(min) and α for system 2 isgreater than α_(min), then the MPE level for system 2 is also greaterthan for system 1.

-   -   (c) If α for system 1 was less than α_(min) and α for system 2        is also less than α_(min), then the MPE level for system 2 is        the same as for system 1.

By means of one or both of these measures, it is possible for anilluminator to meet Class 1 AELs (i.e. be eye-safe) even when the sourceitself is rated as high as Class 4. If the illuminator does not meetClass 1 AELs, with or without these measures, it is still possible for aPL imaging system as a whole, or such system integrated into aproduction line or other wafer/cell handling system, to meet Class 1AELs without resorting to stringent laser safety measures such as safetyshutters and interlocks. This represents a significant simplificationfor the system integration; for example the configuration shown in FIG.3 would be simplified considerably if the light-tight shutters 22 werenot required and the measurement chamber 24 did not have to enclose theimaging system on all sides. Instead, the PL system itself or theproduction line guarding may provide some minimum human access distancefrom the illuminator, and the PL system can prevent direct viewing ofthe illuminator output, i.e. viewing will be limited to reflections froma wafer or solar cell or some object in the PL system or productionline. Reflections off sample edges are of particular concern, sincebroken wafers may present mirror-like edge surfaces at unpredictableangles. Reducing the illuminator brightness by increasing the divergenceangle of the excitation light (approach (i) described above) isparticularly useful in combination with measures that provide a minimumhuman access distance. All these details need to be considered indetermining if a PL imaging system meets Class 1 AELs.

To summarise, it is preferred for a PL imaging system as a whole, orsuch system integrated into a production line or other wafer/cellhandling system, to meet Class 1 AELs without resorting to stringentlaser safety measures such as safety shutters and interlocks. Morepreferably, the illuminator meets Class 1 AELs. With these light safetyconsiderations in mind, we now turn to the description of certainpreferred embodiments of PL imaging systems for in-line inspection ofsilicon solar cell samples. For both area illumination schemes and lineillumination schemes, the above described approaches can be applied toreduce light safety requirements.

In a first embodiment, referred to hereinafter as the ‘flash lamp’approach and illustrated schematically in FIG. 5, a substantial area(preferably at least 1 cm×1 cm, more preferably the entirety) of asample 4 moving through a measurement zone on a transport belt 36 isilluminated with a short pulse of excitation light 8 from one or morehigh intensity sources 50 such as a xenon flash lamp or a pulsed LED,and the resulting PL emission 2 from that area acquired with an areacamera 10. In one specific example, a Broncola ring flash C produces a 1millisecond pulse that, after passing through an excitation filter 15 (a6 mm thick KG1 Schott glass short pass filter), illuminates a siliconsample with an intensity of 10-100 W/cm² (100 to 1000 Suns), and animage of the PL emission acquired with a 1 Megapixel silicon CCD camera.The system may be surrounded by a cylindrical reflector 52 if greaterillumination intensity on the sample is required. The system can alsoinclude collection optics 11 and a long pass filter 14 as in the FIG. 1system, and a shroud 54 to prevent excitation light entering the camera.We note that although the overall speed of the system may be limited bythe camera readout time, depending on the camera technology, this doesnot affect its ability to acquire PL images of moving samples withminimal blurring. In the context of the present invention this is theprimary advantage of high intensity, short pulse illumination, highintensity illumination, for example up to 1000 Suns (100 W/cm²) alsoprovides surprising benefits for PL image clarity and for identifyingcertain defects.

In the embodiment illustrated in FIG. 5, the flash lamp 50 isring-shaped with the camera 10 centrally mounted, enabling both to bepointed orthogonally to the surface of a sample for greater illuminationand imaging uniformity compared to configurations such as those shown inFIG. 1 where one or both of the illumination source 6 and camera 10 isangled with respect to the surface of the sample 4. This arrangement,shown schematically in plan view in FIG. 6, also has the benefit ofallowing an overall more compact system and, more importantly, thecamera and flash lamp can both be closer to the sample withoutobstructing the field of view or casting a shadow. Having the flash lampand camera closer to the sample will generally improve the efficiency ofboth the illumination and PL collection systems.

In an alternative embodiment illustrated in FIG. 7, a dichroic mirror 43is used to introduce the illumination 8 from a flash lamp 50 (or anyother source of illumination suitable for generating PL) into theimaging optical system, so that the working distance can be optimisedentirely for the imaging system. To mitigate the possibility of any PLgenerated from the dichroic mirror itself reaching the camera 10, it ispreferable for the dichroic mirror to be placed well away from thesample so that the collection optics 11 do not effectively focus anysuch PL emission into the camera. If the system of collection optics hasmultiple elements as shown in FIG. 7, an advantageous position for thedichroic mirror is close to the first optical element 11 a.

If the excitation light is from a broad band source such as a flashlamp, the excitation filter 15 becomes an important component because ofthe necessity to prevent longer wavelength excitation light (overlappingthe PL emission band) from reaching the camera. Although dielectricfilters have sharper transitions from high to low transmission thanabsorption filters, which is especially important for indirect band gapmaterials where the PL emission is orders of magnitude weaker than theillumination, their transmission has a strong angular dependence causingthe cut-on/cut-off wavelength to vary with incidence angle. Thecoherent, directional emission from lasers is readily collimated forefficient filtering with dielectric filters, but this is much moredifficult to achieve with the incoherent and essentially isotropicemission from flash lamps or LEDs, favouring absorption filters (such asthe KG1 Schott glass filter mentioned above) or a combination ofabsorption and dielectric filters. We note that lamps that emit over anarrow wavelength range, such as low pressure sodium lamps that emit anextremely narrow doublet around 590 nm, may be advantageous in that theillumination can be easily separated from the silicon PL emission.

Apart from having less abrupt transitions from high to low transmission,absorption filters may also suffer from a heating problem, especiallyfor the in-line inspection of solar cells/precursors where the flashlamp may need to be activated at a frequency of order 1 Hz or higher toinspect every sample. There are several possible ways for dealing withsuch a heating problem, including efficient air or liquid cooling of asolid absorption filter, and using liquid filters where an absorbingliquid is re-circulated through a flow cell, composed of glass forexample, and if necessary through a heat exchanger. Solutions of organicdyes, for example a combination of the IRA 955 and IRA 1034 infraredabsorbers from Exciton, Inc, may be suitable for removing excitationlight in the PL emission band. UV stability of organics may be an issuewhen filtering flash lamp emission, but most UV light can be blockedwith a judicious choice of glass flow cell material or addition of UVabsorbing material in the filter or in the cooling liquid if used, andin any event the optimal solution for a given system of flash lamp,sample material and camera technology may well involve a combination offilters and cooling techniques.

In systems with flash lamp or other short pulse excitation, the imageacquisition time will be determined by the overlap of the illuminationtime and the camera shutter time, and to minimise blurring it isgenerally advantageous for both to be short. In addition theillumination time (pulse duration) should be short to reduce powerconsumption and avoid excessive sample heating, bearing in mind thathigh illumination intensity is generally required to generate sufficientPL signal within a short acquisition time. Leaving the camera shutteropen too long may cause image blurring if the radiative lifetime issufficiently long for the sample to move a significant distance (e.g. bya distance corresponding to one or two camera pixels) before the PLemission has decayed, although this is only likely to be a problem forvery high carrier lifetime samples such as passivated monocrystallinesilicon where the lifetime can exceed several milliseconds. This effectis expected to be negligible for typical multicrystalline silicon waferswhere the carrier lifetime is of order hundreds of microseconds at most.

For preference, the illumination will be provided by a pulsed excitationsource with which the camera shutter is substantially synchronised. Theexcitation source may for example be a xenon flash lamp, a halogen flashlamp, a photographic flash, an LED or a laser, singly or in an array,with a wavelength range suitable for exciting band-to-band PL from thesample. Preferably the illuminator should be eye-safe to minimise lightsafety requirements. More preferably the illumination should beincoherent, i.e. the illumination source should not be a laser, althoughas mentioned previously incoherent illumination is not necessarilyeye-safe. We note that flash lamps (and to a lesser extent LEDs) areadvantageous in this regard because they are extended sources, implyingthat their emission cannot be focused to a point on the retina or,equivalently, limiting the minimum retinal image size. In preferredembodiments the image acquisition time is sufficiently short that thesample moves by a distance of no more than that correspondingapproximately to one row of pixels in the imaging camera. This guidelinedepends on the speed of movement of the samples and on the number ofpixel rows in the camera, but by way of example only, for a process linethroughput of 1 wafer per second and a 1 megapixel camera (1024×1024pixels), this guideline would suggest an image acquisition time ofduration 1 millisecond (ms) or less, which is of order a thousand timesless than the time permissible if the wafer were to be stopped formeasurement. This thousand-fold decrease in acquisition time needs to becompensated by a thousand-fold increase in measurement speed,measurement speed being defined as the luminescence signal, quantifiedfor example as counts per pixel, detected per second. It will be seenthat the requisite increase in measurement speed can be provided by somecombination of increased illumination intensity, improved PL collectionefficiency and different camera technologies and operation.

In a second embodiment, referred to hereinafter as the ‘line scan’approach, a line camera (e.g. silicon or indium gallium arsenide) isused instead of an area camera, and a 2-dimensional PL image acquiredline-by-line as the sample passes through the measurement zone beforethe line camera. The illumination can be broad area but for efficiencypurposes it is preferable to illuminate only the linear portioncorresponding to the line camera's view, preferably with some degree of‘over-filling’ of the illumination so that the illuminated and imagedareas don't have to be precisely aligned. A suitable system is shownschematically in FIGS. 8A (plan view) and 8B (side view), where theexcitation source 6, line camera 28 and associated optics have beenomitted from the plan view for clarity. The system includes focusingoptics 30 to focus the excitation light 8 onto the sample 4 such thatthe illuminated portion 32 is coterminous with or slightly wider thanthe imaged portion 34, and collection optics 11 to image the PL emission2 onto the line camera, as well as various other components(homogenisation optics 12, long pass filter 14, excitation filter 15 andcomputer 16) if required, similar to the FIG. 1 system. In thisparticular example the width 33 of the illuminated portion 32 isapproximately three times larger than the width 35 of the imaged portion34, corresponding to a one pixel margin either side of the imagedportion. However if the available excitation power is a limiting factor,the illuminated and imaged portions can be substantially coterminous.The sample is moved through the measurement zone on transport belts 36,from left to right in this case as indicated by the arrow 38, so thatthe illuminated and imaged portions are effectively scanned across thesample. The sample illumination will generally be continuous rather thanpulsed, and the excitation source may for example be an array of LEDs orlasers, although for preference it is an incoherent source, i.e. notlaser-based, and the illuminator as a whole is eye-safe. We note thatfrom a light safety perspective line illumination systems can beadvantageous because the eye cannot focus both axes simultaneously,limiting the minimum retinal image size. The illuminated and imagedportions need not be oriented perpendicularly to the direction of motion38 as shown in FIG. 8A, so long as they extend across the full width ofthe sample or at least the full width of the sample area that needs tobe measured. However a substantially perpendicular orientation minimisesthe area to be illuminated, thereby minimising the power requirement forthe excitation source, and is therefore to be preferred.

Another suitable system is shown schematically in side view in FIG. 9.In this case the transport belts 36 have a gap or unobstructed region 39to allow the excitation source 6 and line camera 28 to be on oppositesides of the sample 4. In this particular embodiment the sample isilluminated through the gap, but in an alternative embodiment the PLemission 2 could be imaged through the gap. It will be appreciated thatas a result of continued movement across the gap, the entire sample willbe progressively unobstructed for illumination or imaging. In yetanother alternative embodiment, suitable for samples that need not beanalysed across their entire width (similar to the situation shown inFIG. 4), the gap in the transport belts is omitted.

To maximise the spatial resolution of the ‘line scan’ approach, thewidth of the imaged portion 34 should correspond to one row of pixels inthe line camera 28. To achieve the same spatial resolution as would beobtained from a 1 Megapixel area camera, it is necessary to compensatefor a thousand-fold decrease in PL signal acquisition time with athousand-fold increase in measurement speed. This is essentiallyequivalent to the measurement speed increase for the ‘flash lamp’approach described above.

We turn now to a more detailed discussion of the illumination andimaging parts of a line scan PL imaging system. As shown in FIGS. 10Aand 10B (side view and plan view respectively), a line illuminator cancomprise an optical fibre coupled laser or LED array 56 and focusingoptics 30 comprising a pair of cylindrical lenses 58, 60 with dimensionsand focal lengths chosen according to the requirements of the specificsystem. As shown in FIG. 10A, excitation light 8 emerging from theoptical fibre 62 is collimated in one direction by the first cylindricallens 58, then focused to a line 64 by the second cylindrical lens 60. Asshown in FIG. 10B the excitation light 8 continues to diverge in theorthogonal direction, to cover the full width of the sample. Thisarrangement tends to produce an approximately Gaussian intensitydistribution along the line 64, determined largely by the optical fibreoutput, which is acceptable for PL imaging since it can be correctedwith a calibration procedure, so long as the illumination intensity inthe outer regions is sufficient to produce a measurable PL response.Line illuminators that produce more uniform intensity distributions arealso known in the art.

Turning now to consideration of collection optics, one possible system,comprising an arrangement of four cylindrical lenses with dimensions andfocal lengths chosen according to specific apparatus requirements, isillustrated in FIGS. 11A (side view) and 11B (plan view). PL emission 2from the illuminated line 64 is collimated in one direction with a firstcylindrical lens 72, demagnified with a pair of cylindrical lenses 74,76 in a ‘beam expander’ configuration, and focused onto the line camera28 with a fourth cylindrical lens 78. A number of other possible systemsof collection optics with varying degrees of complexity will occur tothose skilled in the art, using components such as lenses, mirrors andoptical waveguides. For example FIG. 12 illustrates schematically asystem where an array 68 of widely spaced optical fibres collects PLemission 2 from an illumination line 64, optionally with the aid oflenses, and guides it to a line camera 28. Generally speaking the choicewill be informed by factors such as cost (with off-the-shelf opticalcomponents preferred over custom-made components) and the requiredcollection efficiency; for example if the PL signal is relativelystrong, or if a high sensitivity camera is used, a standard camera lensmay suffice.

An important difference between ‘line scan’ imaging (illustrated inFIGS. 8A, 8B and 9) and the more conventional area imaging (illustratedin FIGS. 1 and 5 for example) is that the light gathering portion of thecollection optics (lenses 72 and 74 in FIGS. 11A and 11B) can be locatedquite close to the sample without obscuring the illumination, greatlyenhancing the collection efficiency (which may be defined as the number(or rate) of luminescence photons that are detected divided by thenumber (or rate) of luminescence photons emitted by the sample into ahemisphere). Depending on the design details, we estimate that thecollection optics shown in FIGS. 11A and 11B are two or three orders ofmagnitude more efficient than the typical collection optics of aconventional ‘area imaging’ (FIG. 1) PL system, quantified below in the‘baseline example’.

Similarly, the illuminator can be located quite close to the samplewithout obscuring the PL collection optics, which can be advantageousfor light safety. To explain with reference to FIG. 10B, the closer theilluminator is to the sample, the greater the divergence of theexcitation light 8 impinging on the sample, reducing the brightness ofthe illuminator.

Under quasi steady state conditions, a reasonable approximation even formillisecond-level illumination times as may be used with a flash-lampfor example since the minority carrier lifetime in silicon as used inthe solar industry is typically of order 10 to 100 μs, the minoritycarrier concentration An (which affects the PL intensity) and thegeneration rate G (determined by the illumination intensity among otherthings) are related by the equation Δn=G*τ where τ is the minoritycarrier lifetime. From this it follows that, for a given illuminationintensity, a stronger PL signal will be obtained from a sample with alonger carrier lifetime, e.g. monocrystalline silicon compared tomulticrystalline silicon, or a passivated silicon wafer (in later stagesof a solar cell line) compared to a raw silicon wafer. Although theminority carrier lifetime τ can only be considered to be constant at lowinjection levels (i.e. low Δn), in general a stronger PL signal willalso result from more intense illumination, i.e. larger generation rateG, which is an important aspect of the ‘flash lamp’ approach describedpreviously.

As described in PCT patent application No AU2010/001045 entitled‘Photoluminescence imaging systems for silicon photovoltaic cellmanufacturing’, incorporated herein by reference, it is sometimespreferable when performing PL imaging of silicon samples to use a cameratechnology such as indium gallium arsenide (InGaAs) that, unlikesilicon-based cameras, is sensitive across the entire silicon PLemission spectrum. All other things being equal, we estimate thatreplacing a silicon camera with an InGaAs camera improves measurementspeed by some 20×. Measurement speed can also be improved by usingcameras with larger pixels or, at the expense of spatial resolution,binning pixels.

A third embodiment, somewhat similar to the ‘line scan’ approach, uses atime delay integration (TDI) camera. A TDI camera can be thought of asan integrated array of line cameras, e.g. 96 or 128 lines of 1024 pixelson a single chip, typically using the same silicon CCD technology as inconventional line or area cameras. TDI cameras are well suited foracquiring images of a moving sample, with the direction of movementperpendicular to the pixel lines: as the sample is moved, the chargefrom the detected signal is transferred to the next pixel line andaccumulated, with synchronisation of the transport speed and the chargetransfer. Consequently, a TDI camera with N pixel lines measures thesignal from a given portion of a sample N times, improving thesignal-to-noise ratio by a factor of √N compared to a line camera forthe same total measurement time. A suitable system for this ‘TDI camera’approach is shown schematically in side view in FIG. 13, and it will beseen that the configuration is quite similar to the line scan systemshown in FIG. 8B but with a TDI camera 40 in place of the line camera.FIG. 13 also shows a transport belt drive unit 42 controlled by thecomputer 16, in accordance with the need to synchronise the motion ofthe sample 4 with the TDI camera operation. A significant but lessobvious difference with the line scan system is that for a TDI camerawith N pixel lines, the imaged portion 34 needs to be N times wider thanfor the line scan system. With a one pixel margin either side of theimaged portion as for the ‘line scan’ configuration shown in FIGS. 8Aand 8B, the illuminated portion 32 will be a factor of (N+2)/3 widerthan in the line scan configuration; alternatively the illuminatedportion will be N times wider than in the line scan configuration if theilluminated and imaged portions are coterminous. Either way, for a givenexcitation source 6 the illumination intensity on the sample will bereduced by the same factor, and when designing a TDI system this needsto be considered against the N times longer signal acquisition timeadvantageously provided by the N pixel lines. Finally, it will beappreciated that the variant configurations discussed above for linecamera systems, e.g. the configuration shown in FIG. 9, are alsoapplicable to TDI systems.

EXAMPLES

In this section a ‘baseline example’ of an optical system is provided,such as may be found in a prior art PL imaging system with full fieldimaging onto an area camera, illustrated schematically for example inFIG. 1 or 2, with which all subsequent examples are to be compared. Itis assumed that measurement noise is dominated by statistical noise,i.e. the signal to noise is given as the square root of the total numberof counts, and define 2000 counts per pixel as the target for PLmeasurements that have sufficient signal to noise. Several measurementparameter/sample/hardware combinations will be outlined that yield thistarget, and it will be observed that there are general rules that can beused to derive alternative combinations. In each case the sample will bea 156 mm×156 mm, 200 μm thick 1 Ω·cm p-type silicon wafer, but thespecific material quality will vary between three possibilities:

-   -   As-cut unpassivated multicrystalline silicon, with an effective        carrier lifetime of 0.5 to 2 μs (worst case′)    -   Passivated or diffused multicrystalline silicon, with an        effective carrier lifetime of around 10 μs    -   High lifetime passivated monocrystalline silicon, with an        effective carrier lifetime of around 1 ms (best case′)

Baseline Example

Referring to FIG. 1, a 156×156 mm² sample 4 is illuminated with 750 nmlight at an on-sample intensity of 100 mW/cm² (1 Sun), and thephotoluminescence 2 focused onto a Si CCD-based 1 Megapixel (1024×1024pixels) camera 10 having 5×5 μm² pixels using collection optics 11comprising an F#=2.8 lens with a focal length f=25 mm. To estimate thecollection efficiency, it is noted that to focus the entire sample areaonto the camera chip, the collection optics needs to have amagnification M=1024×5e-3/156=0.033, requiring an object distance (i.e.the distance between the lens and the sample) of O=(1+1/M)*f=787 mm.Noting that the aperture of the lens is given by D=f/#=25 mm/2.8=8.9 mm,the acceptance area of the lens will be πD²/4=63 mm². Comparing thiswith the surface area of a hemisphere of radius O (2πO²), a collectionefficiency of 0.0016% is calculated.

With this system, it is estimated that an unpassivated multicrystallinesilicon sample will yield between 2 and 6 counts per second per pixel,implying a total measurement time of between 330 and 1000 s to achieve2000 counts. In comparison, because of the longer carrier lifetime anestimated 2000 counts could be achieved in about 60 s or 600 ms forpassivated multicrystalline silicon or high lifetime passivated siliconsamples respectively.

Example 1 2D Area Imaging Geometry in Three-Belt System of FIG. 3(Stationary Sample)

To incorporate a three-belt system into a solar cell line with athroughput of one cell per second, the measurement time clearly cannotexceed one second. To guarantee this for unpassivated samples it isnecessary to achieve a thousand-fold increase in measurement speed. Thiscould be done for example with a combination of:

-   -   1) Si-CCD camera with 20×20 μm² pixels (16× gain compared to the        baseline example)    -   2) F#=2 lens (2× gain)    -   3) Illumination intensity of 1 W/cm² i.e. 10 Suns (10× gain)    -   4) 2×2 pixel binning (4× gain)        It will be appreciated that many other combinations are        possible, e.g. 40 Suns illumination without pixel binning

Example 2 Line Scan System with Line Illumination (165 μm Width and 156mm Length) and Detection on Unpassivated Wafer

In this case a 1 ms measurement time per line is needed, requiring a 10⁶times increase in measurement speed compared to the Baseline Example.One possible combination is:

-   -   1) InGaAs camera (20× gain)    -   2) 25×25 μm² pixels (25× gain)    -   3) Illumination intensity of 1 W/cm² i.e. 10 Suns (10× gain)    -   4) Improved collection efficiency (200× gain)        It should be noted that because the illuminated area is much        smaller than in the Baseline Example, it would be relatively        straightforward to use much higher illumination intensities        (hundreds of Suns) if collection efficiency improvements are        more limited.

Example 3 Line Scan System with Line (165 μm Width and 156 mm Length)Illumination and Detection on Passivated Multicrystalline Wafer

As for Example 2 a 1 ms measurement time per line is needed, which inthis case requires a 60,000 times increase in measurement speed comparedto the Baseline Example. One possible combination is:

-   -   1) Si-CCD camera with 20×20 μm² pixels (16× gain)    -   2) Illumination intensity of 2 W/cm² i.e. 20 Suns (20× gain)    -   3) Improved collection efficiency (200× gain)        Alternatively an InGaAs camera would give the required        measurement speed if collection efficiency improvements are more        limited, or if the PL intensity increases sub-linearly with        illumination intensity.

Example 4 Line Scan System with Line (165 μm Width and 156 mm Length)Illumination and Detection on High Lifetime Passivated Wafer

In this case a 600 times increase in measurement speed is required toachieve a 1 ms measurement time. One possible combination is:

-   -   1) Si-CCD camera with 20×20 μm² pixels (16× gain)    -   2) Illumination intensity of 0.2 W/cm² i.e. 2 Suns (2× gain)    -   3) Improved collection efficiency (20× gain)        The increase in measurement speed could alternatively be        achieved with the baseline geometry, i.e. without any        modification of the imaging system to improve the collection        efficiency, if an illumination intensity of 40 Suns were used.

Example 5 Flash Based System with Broad Illumination Spectrum (500-800nm) on Unpassivated Wafer

A 1 ms measurement time is needed, i.e. a 10⁶ times improvement comparedto the Baseline Example. Assuming that the response of an unpassivatedwafer to that spectrum is on average the same as for 750 nm excitation,one possible combination is:

-   -   1) InGaAs Camera (20× Gain)    -   2) 25×25 μm² pixels (25× gain)    -   3) Illumination intensity of 50 W/cm² i.e. 500 Suns (500× gain)    -   4) F#=1.4 lens or 2×2 pixel binning (4× gain)        It will be noted that an extremely high illumination intensity        is used.

Example 6 Flash Based System with Broad Illumination Spectrum (500-800nm) on Passivated Multicrystalline Wafer

A 1 ms measurement time is needed, i.e. a 60,000 times improvementcompared to the Baseline Example. One possible combination is:

-   -   1) Si-CCD camera with 20×20 μm² pixels (16× gain)    -   2) Illumination intensity of 10 W/cm² i.e. 100 Suns (100× gain)    -   3) F#=1.4 lens (4× gain)    -   4) 3×3 pixel binning (9× gain)        Using an InGaAs camera or improved collection efficiency would        allow lower illumination intensities; these options would also        be useful if the PL intensity increases sub-linearly with        illumination intensity.

Example 7 Flash Based System with Broad Illumination Spectrum (500-800nm) on High Lifetime Passivated Wafer

A 1 ms measurement time is needed, i.e. a 600 times improvement comparedto the Baseline Example. One possible combination is:

-   -   1) Si-CCD camera with 20×20 μm² pixels (16× gain)    -   2) Illumination intensity of 2 W/cm² i.e. 20 Suns (20× gain)    -   3) F#=2 lens (2× gain)        This can be achieved with the baseline geometry, i.e. without        any modification of the imaging system to improve the collection        efficiency.

Example 8 TDI Camera System with 128 Lines on Unpassivated Wafer

2000 counts per pixel is needed with 128 ms exposure time for each partof the wafer, implying an 8000 times improvement in measurement speedcompared to the Baseline Example. One possible combination is:

1) Si-CCD camera with 20×20 μm² pixels (16× gain)

-   -   2) Illumination intensity of 1 W/cm² i.e. 10 Suns (10× gain)    -   3) Improved collection efficiency (50× gain)        Alternatively an InGaAs camera can provide the required        measurement speed if collection efficiency improvements are more        limited.

Example 9 TDI Camera System with 128 Lines on PassivatedMulticrystalline Wafer

2000 counts per pixel are needed with 128 ms exposure time for each partof the wafer, implying a 400 times improvement compared to the BaselineExample. One possible combination is:

1) Si-CCD camera with 20×20 μm² pixels (16× gain)

-   -   2) Illumination intensity of 1 W/cm² i.e. 10 Suns (10× gain)    -   3) 2×2 pixel binning (4× gain)

Example 10 TDI Camera System with 128 Lines on High Lifetime PassivatedWafer

2000 counts per pixel are needed with 128 ms exposure time for each partof the wafer, implying a 4 times improvement compared to the BaselineExample. This can be achieved simply by using 4 times the illuminationintensity or 2×2 pixel binning

In the above-described preferred embodiments, semiconductor samples havebeen subjected to a photoluminescence analysis in the form of PLimaging, i.e. the acquisition of a 2-dimensional image of thephotoluminescence generated from a substantial area of each sample, witha view to deriving spatially resolved information on one or morematerial properties. However other forms of photoluminescence analysisare also within the scope of the present invention. For example the PLemission from the illuminated area could be fed into a spectrometer toanalyse the spectral content, say to look for a PL band indicative of animpurity. In another example the total PL emission signal could bemeasured to yield information on the average value of a sample property,with the averaging being performed line-by-line or across the entireilluminated area, or to provide a rapid method for identifying defective(e.g. shunted) solar cells during or after manufacture.

Although the present invention has been described with particularreference to certain preferred embodiments thereof, variations andmodifications of the present invention can be effected within the spiritand scope of the following claims.

1. A method for analysing a sample of a semiconductor material, saidmethod comprising the steps of: passing said sample to a measurementzone; applying an illumination to said sample to produce aphotoluminescence response from said semiconductor material; andconducting at least a photoluminescence analysis of said sample in saidmeasurement zone while maintaining motion of said sample, wherein saidphotoluminescence analysis comprises providing a multi-pixel imagecapture device for acquiring an image of the photoluminescence emittedfrom said sample.
 2. A method according to claim 1, wherein said methodis performed with a system that meets Class 1 accessible emission limits(AELs) without said measurement zone being enclosed.
 3. A methodaccording to claim 1, wherein the illumination source, the image capturedevice or an optical element associated with the illumination source orthe image capture device moves or pivots to follow the motion of saidsample.
 4. A method according to claim 1, wherein said analysis isperformed in less than 1 second.
 5. A method according to claim 1,wherein said illumination is applied to an area of said sample and saidimage is acquired with an area image capture device in an acquisitiontime t, wherein said sample is moving at a speed v relative to said areaimage capture device, and wherein the product of the acquisition time t(s) and the speed v (m·s⁻¹) is less than a distance on said samplecorresponding to one row of pixels in said area image capture device. 6.A method according to claim 5, wherein said method is applied to asample of a semiconductor material comprising silicon, and saidphotoluminescence is generated with an illumination intensity betweenabout 10 and 100 W·cm⁻².
 7. A method according to claim 5, wherein saidillumination is applied to the entire area of a surface of said sample,and said area image capture device acquires an image of thephotoluminescence emitted from said entire area.
 8. A method accordingto claim 1, wherein said illumination is provided by a source positionedso as to illuminate a first portion of said sample and said imagecapture device is positioned so as to detect photoluminescence emittedfrom a second portion of said sample, wherein said first portion andsaid second portion are at least partially overlapping, and wherein saidmethod further comprises the steps of: moving said sample relative tosaid source and to said image capture device such that said secondportion is scanned across a substantial area of said sample; andinterrogating said image capture device repeatedly to acquire an imageof the photoluminescence emitted from said area.
 9. A method accordingto claim 8, wherein said image of the photoluminescence is acquired witha line camera or a time-delay integration camera.
 10. A method accordingto claim 8, wherein said first portion is wholly within said secondportion.
 11. A method according to claim 8, wherein said source providesbroad area illumination of said sample.
 12. A method according to claim8, wherein said first and second portions extend across a substantialfraction of a dimension of said sample, said dimension beingsubstantially perpendicular to the direction of movement of said sample.13. A system for conducting an analysis of a sample of a semiconductormaterial, said apparatus comprising: a transport mechanism fortransporting said sample to a measurement zone; analysis equipment forconducting at least a photoluminescence analysis of said sample withinsaid measurement zone, said analysis equipment comprising a source ofpredetermined illumination suitable for generating photoluminescencefrom said semiconductor material, and a multi-pixel image capture devicefor acquiring an image of the photoluminescence emitted from saidsample; and motion apparatus to maintain motion of said sample withinsaid measurement zone during said analysis.
 14. A system according toclaim 13, wherein said system is adapted such that, in use, said systemmeets Class 1 accessible emission limits (AELs) without said measurementzone being enclosed.
 15. A system according to claim 13, wherein saidsource, said image capture device or an optical element associated withsaid source or said image capture device is capable of moving orpivoting to follow the motion of said sample.
 16. A system according toclaim 13, wherein said source of predetermined illumination is adaptedto illuminate an area of said sample, and said image capture device isan area image capture device adapted to capture an image of saidphotoluminescence in an image acquisition time t, wherein said motionapparatus moves said sample at a speed v relative to said area imagecapture device such that the product of the image acquisition time t (s)and the speed v (m·s⁻¹) is less than a distance on said samplecorresponding to one row of pixels in said area image capture device.17. A system according to claim 16, wherein said system is configured toconduct an analysis of a sample of a semiconductor material comprisingsilicon, and said source is configured to generate photoluminescencewith an illumination intensity between about 10 and 100 W·cm⁻².
 18. Asystem according to claim 16, wherein said source is adapted toilluminate the entire area of a surface of said sample, and said areaimage capture device is adapted to acquire an image of thephotoluminescence emitted from said entire area.
 19. A system accordingto claim 13, wherein said source is adapted to illuminate a firstportion of said sample, and said image capture device is adapted todetect photoluminescence emitted from a second portion of said sample,wherein said first portion and said second portion are at leastpartially overlapping, and wherein said analysis equipment furthercomprises an interrogation module for interrogating said image capturedevice repeatedly while said motion apparatus moves said sample suchthat said second portion is scanned across a substantial area of saidsample, to acquire an image of the photoluminescence emitted from saidsubstantial area.
 20. A system according to claim 19, wherein said imagecapture device comprises a line camera or a time-delay integrationcamera.
 21. A system according to claim 19, wherein said first portionis wholly within said second portion.
 22. A system according to claim19, wherein said source is adapted to provide broad area illumination ofsaid sample.
 23. A system according to claim 19, wherein said first andsecond portions extend across a substantial fraction of a dimension ofsaid sample, said dimension being substantially perpendicular to thedirection of movement of said sample.